The role of cyclic AMP (cAMP) in the regulation of tumor growth is unclear. Following the administration of dibutyryl cAMP (dbcAMP), certain animal tumors cease growing or actually regress, suggesting that cAMP exerts an inhibitory influence on tumor growth but other tumrs, although similar histologically, are unresponsive. It has been suggested that in the resistant tumors, the unresponsiveness is due to a decrease in the capacity of the intracellular receptors to bind cAMP. Consistent with this hypothesis, a dbcAMP resistant variant of the Morris hepatoma has been found to have less capacity to bind cAMP than do Morris hepatoma variants that are inhibited by dbcAMP. It is possible that in certain tumors, unregulated replication is due in part to failure of inhibition of cell growth by endogenous cAMP, due to impairment of intracellular cAMP binding. We postulate that in the dbcAMP resistant tumors the impairment in binding may be severe, while in the dbcAMP responsive tumors there may be only a partial impairment in binding, with the result that relatively large amounts of exogenous dbcAMP can inhibit growth. In preliminary studies, we have examined the cAMP binding proteins in the nuclei and cytoplasm of normal rat liver, and have observed that cAMP binding in the nucleus is enhanced by ATP, ADP, GTP and GDP and decreased by 5'AMP and adenosine. The effect of these nucleotides on cAMP binding in the cytosol differed from that in the nucleus. We have also performed preliminary studies on the cytosol and nuclei from a dbcAMP responsive and nonresponsive hepatoma, and have observed differences in the effect of these nucleotides on nuclear cAMP binding. In this project we will isolate the individual hepatic nuclear and cytosolic binding proteins, in order to characterize further their responses to adenine and guanine nucleotides, and to determine which have protein kinase activity. In the second part of the project we intend to study the cAMP binding proteins in the nuclei and cytoplasm from: (1)\other dbcAMP responsive and nonresponsive hepatomas grown in vivo in order to determine whether our initial observations are observed in other tumors; and (2)\rat hepatomas grown in tissue culture in order to rule out cell heterogeneity as the reason for our observations.